Process in the electrolytic deposition of chromium, and the preparation of solutions therefor



Patented Mar. 5, 1935 UNITED STATES PROCESS IN THE ELECTROLYTIC DEPOSI-TION OF CHROMIUM, AND THE PREPARA- TION F SOLUTIONS THEREFOR Wilfred W.Scott, Attilio A. Bissiri, and William 0. Gregory, Los Angeles, Calif.;Harriet E. Scott executrix of said Wilfred W. Scott, de-

- ceased No Drawing. Application April 22, 1932,

Serial No. 607,012

6 Claims.

In the electrolytic deposition of chromium, theuse of chromic acid asthe source of chromium ions has been universally accepted. It is a wellrecognized scientific fact that if an ordinary low voltage, directcurrent, is used in the process of chromium electroplating, a brightsurface of metallic chromium can not be deposited on the cathode unlesssome other substance in addition to chromic acid and water, is used inthe electrolytic bath. The use of such addition agents has beenaccompanied in some cases by a certain degree of success from thestandpoint of commercial application, but all such processes at presentin use are at their best more or less unreliable, causingelectroplaters, at times, considerable trouble.

Our invention provides an entirely new process which is a greatimprovement over all other existing processes of chromium plating.

According to our invention the electrolyte is composed of a solution ofchromic acid and varying amounts of selenium or tellurium, or acombination of the two, added in any form that is, or becomes, solublein the water solution of chromic acid. Small amounts of sulphates, asfor instance is sometimes present in coimnercial grades of chromic acid,are not detrimental to the successful deposition of chromium, accordingto our invention.

In introducing the selenium or tellurium in the electrolytic bath, it isimmaterial whether it is as the element, or as a compound of the elementselenium or tellurium. Repeated and careful experiments, both on a smallscale and on a commercial scale, have disclosed the fact that thepresence of even small amounts of either selenium or tellurium resultsin the deposition of crystals of metallic chromium at the'cathode' ofsuch a nature that a beautiful bright and hard finish is obtained. Theexact nature of the chemical reaction or'reactions taking place is verydifi'icult to determine, but it is possible that the addition of eitherselenium or tellurium results in the formation of complex salts ofchromium and selenium, or chromium and tellurium, which complex salts,.when present even in small' amounts, bring about the desired crystalformation of metallic chromium at the cathode.

When tellurium is used, the addition of a small amount of seleniumnmakespossible the desired good results using a total amount of selenium andtellurium which is less than the amount that would have been required ifonly tellurium had been used. The'same condition has proved to hold truewhen a small amount of tellurium is used in conjunction with theselenium.

The actual results have shown also a decided improvement in the throwingpower.

The process of our invention is characterized 5 by a high degree offlexibility as to what concerns temperature of the solution, currentdensity, concentration of chromic acid, and the ratio of selenium ortellurium to chromic acid. A ratio of selenium or tellurium to chromicacid 10 varying from a minimum of 1 part of selenium or tellurium to10,000 parts of chromic acid, and a maximum of 1 part of selenium ortellurium to 50 parts of chromic acid, has been found to give thedesired results. However, the optimum l5 ratio is obtained when aminimum of 1 part of selenium or tellurium to 1000 parts of chromicacid, and a maximum of 1 part of selenium or tellurium to 250 parts ofchromic acid is observed. tion can be easily obtained by empiricalmethods, thus greatly simplifying the process, and increasing itscommercial value. When a new solution is made, the preferred ratio ofselenium or tellurium to chromic acid is obtained by weighing thematerials, or measuring them if in solution. but after that, thephysical properties (brightness and hardness) of the objects plated willtell the operator when more selenium or tellurium must be added. Ifdesired, ordinary methods of quantitative analysis may also be used.

What is claimed is:

1. A process which includes the step of depositing chromiumelectrolytically from a solution of chromic acid, in the presence of anamount of compounds of selenium or tellurium dissolved in the chromicacid to provide a proportion of one part of selenium or tellurium tofrom 250 to 1000 parts of chromic acid.

2. A process which includes the step of electro- 40 depositing chromiumfrom a chromic acid solution in which is dissolved a compound orcompounds of selenium providing a ratio of 1 part of selenium to from 50to 10,000 parts of chromic. acid.

3. A process which includes the steps of dissolving in a chromic acidsolution a compound or compounds of tellurium to provide the proportionof 1 part of tellurium to from to 10,000 50 parts of chromic acid, withthe optimum proportion being 1 part of tellurium to from 250 to 1000parts of chromic acid and electrodepositing chromium from said solution.

4. A process which includes the step of electro- The control of theelectrolytic solu- 20 depositing chromium from chromic acid in thepresence of both selenium and tellurium.

5. A process which includes the steps of making an electrolytecontaining in solution chromium and both selenium and tellurium andelectrolytically depositing chromium therefrom.

6. A process for the continuous electrolytic deposition of chromium fromchromic acid which includes the introduction into the electrolyte ofboth compounds of tellurium and compounds of selenium in the proportionof 1 part of tellurium and selenium together to from 250 to 1000 partsof chromic acid and the electrolytic deposition of chromium therefrom.

WILFRED W. SCOI'I. ATTILIO A. BISSIRI. WILLIAM C. GREGORY.

